1. Field of the Invention
This invention relates to an apparatus and a method for the chemical deposition of synthetic material on a rod member, in a process for the manufacturing of an optical fibre.
2. Description of the Related Art
As is well known, a process for manufacturing an optical fibre comprises first producing a preform of vitreous material by a chemical deposition process, and then drawing the optical fibre from the preform after having positioned the perform vertically within a furnace, so as to cause the fusion of a lower part thereof.
Patent EP 367871 in the name of Corning Glass Works describes a method for obtaining an optical fibre. This method initially comprises the step of depositing particles of glass comprising base glass and a refractive-index-increasing dopant onto a mandrel. The mandrel is then removed and the resulting soot preform is consolidated so as to form a core preform. The core preform is stretched and the hole in it is closed to form a core rod, or cane. Cladding glass soot is then deposited on the core rod to obtain a final preform, which is then consolidated and drawn so as to obtain an optical fibre.
A process of the type just described is commonly known as an OVD (Outside Vapour Deposition) process. According to patent EP 367871, stretching of the core preform may be performed by means of a traction device comprising a pair of traction driving wheels, which apply a downward traction force to the opposite sides of the core rod being formed.
When the latter step is performed, it is possible that, as a result of an inaccurate positioning of the core rod, of an inhomogeneous thermal profile within the furnace or of the action of the traction wheels, opposite sides of the core rod are subjected to different forces which cause bending of the core rod during its formation; for this reason, core rods that are not perfectly straight can be produced. This shape defect of the core rod will be referred to as an “intrinsic shape defect”, to distinguish it from other types of shape defects caused by subsequent process steps.
To mitigate the problem of intrinsic shape defects, international patent application WO01/49616 in the name of Pirelli Cavi e Sistemi S.p.A. proposes to impart a twist to the core rod during stretching from the core preform, so as to produce a core rod with a high degree of straightness.
Again in accordance with patent EP 367871, before the cladding glass soot is deposited on the core rod, the core rod is attached at one end to a gripping device mounted on a lathe, so that a rotation motion can be applied to the core rod about a predetermined rotation axis (which will also called “deposition axis”). The Applicant has noted that, when this step is carried out, positioning errors of the core rod on the gripping device can occur, with the result that the axis of the core rod does not coincide with the intended deposition axis. Therefore, when a torque is applied to the core rod to produce rotation thereof about the deposition axis, the rod will not rotate coaxial to deposition axis but on a conic trajectory about the deposition axis.
U.S. Pat. No. 5,958,102 in the name of Shin-Etsu Chemical Co, describes a deposition process wherein one end of the core rod is held by a gripping device and the opposite end is rotatively held by rollers. According to the Applicant, although holding both ends of the rod may force the rod to lie substantially along the deposition axis, a positioning error of the rod on the gripping device may in this case cause bowing of the rod with respect to the deposition axis. Therefore, a misalignment would still exist between the rod axis and the deposition axis.
The Applicant has also noted that another cause of displacement of the deposition rod from the intended deposition axis is represented by possible misalignments among the different components of the device that applies rotation to the deposition rod. This device typically comprises, in addition to a gripper (also referred to as a gripping member or gripping device as previously described), a motor for generating the rotation torque. A defective connection between motor and gripping member can lead to a misalignment thereof, which again can result in an erroneous orientation or in a bending of the deposition rod.
An additional cause of shape defect can be the change in stress in the rod that result from heating the rod during the deposition process, as described in EP 630866.
As a result of the abovementioned straightness and/or orientation errors of the deposition rod, the rod may wobble when rotated during the deposition process, thus determining an uneven deposition of soot and a non-symmetrical final perform. Therefore, the final preform may have a central portion (defined by the core rod) that is not perfectly centred, and the resulting optical fibre will have a core that is not concentric with the cladding.
The core/clad concentricity, defined as the distance between the axis of the core and the axis of the cladding, is a fundamental parameter for an optical fibre. Typically the concentricity should have a small value (e.g. less than 0.5 μm, preferably less than 0.3 μm) so that, when two end portions of two different optical fibres are coupled together, the attenuation in the transmitted light is small. Optical fibres are in fact typically connected by aligning the outer surfaces of the corresponding claddings and therefore, if the cores are not perfectly located along the axes of the corresponding fibres, coupling between the two cores may be partial, giving rise to a coupling with high losses.
Different techniques are known for improving the core/cladding concentricity in optical fibres.
U.S. Pat. No. 5,658,365, in the name of Corning, proposes to apply a longitudinal tension to the deposition rod while holding it during the deposition process. The tensile force, which serves to reduce the wobbling problem, may be applied at one or both ends of the rod. The Applicant observes that the apparatus required for applying such a longitudinal tension during the deposition process is rather complex and expensive.
International patent application WO01/23311, in the name of FIBRE OTTICHE SUD—F.O.S., proposes to grip one end of the deposition rod for imparting rotation thereof, and applying two spaced radial constraints to the opposite end of the rod, thus forcing the rod to lie coaxial with the axis of rotation. The Applicant observes that this technique is more effective in reducing intrinsic shape defects of the deposition rod than in solving problems arising from rod positioning errors or misalignments of the rotation device. In fact, when positioning errors or rotation device misalignments occur, the rod undergoes a stress that tends to cause bending thereof.